A standard motor-vehicle door latch mounted on an outer door edge has a latching mechanism comprised mainly of a fork that can in a locked position engage around a jamb-mounted bolt and hold the door closed, and that can in an unlocked position release this bolt. The latching mechanism also normally has a pawl that is used to block the fork in the locked position, the fork being spring-loaded to move into the unlocked position when released by this pawl.
When the vehicle door is closed with the fork locked in place by the pawl and itself holding the bolt of the doorpost an annular elastic seal all around the door is pressed against the rim of the door opening. The seal is compressed to prevent leakage in both directions, both to keep water out when it is raining and to prevent the door from whistling when the vehicle is moving at high speed. At today's high travel speeds it is therefore necessary to close the door very tightly, thereby highly compressing the door seal.
Accordingly as described in German patent 3,150,621 filed Dec. 21, 1981 by one of the instant inventors it is known to provide each latch with a motor connected via a crank to an operating member that in turn is directly engageable both with the latch fork and with the pawl. To open the door the operating member pulls the pawl off the fork detent and pivots the fork out to release the bolt and to close the door the operating member moves oppositely to oppositely rotate the fork while allowing the pawl to catch on the detents of the fork. Such a system can close the doors very tightly once the door has been pushed substantially to, that is the power mechanism takes care of the final stages of closure.
The main problem with this system is that if the motor or the vehicle's electric supply fails the door cannot be opened manually. Proposed manual overrides are often complex in the extreme and greatly increase the cost of the latch. In addition the manual override often works in a nonstandard manner, not like just using the standard manual latch handle.